Question

Given table shows the $CO_{2}$ compensation point and optimum $CO_{2}$ concentration for photosynthesis for $C_{3}$ and $C_{4}$ plants. Select the correct values for $A$ and $B$.

• A. $A\rightarrow 0 - 50\, ppm, B\rightarrow 300\, ppm$
• B. $A\rightarrow 0 - 10\, ppm, B\rightarrow 450\, ppm$
• C. $A\rightarrow 100 - 150\, ppm, B\rightarrow 250\, ppm$
• D. $A\rightarrow 100 - 110\, ppm, B\rightarrow 290\, ppm$

Answer 1

Answer: (B)

$A\rightarrow 0 - 10\, ppm, B\rightarrow 450\, ppm$

Answer 2

$CO_{2}$ compensation point or threshold value is that concentration of $CO_{2}$ at which illuminated plant parts stop absorbing carbon dioxide from their environment. At this value, $CO_{2}$ fixed in photosynthesis is equal to $CO_{2}$ evolved in respiration and photorespiration. The value is $25-100 \,ppm$ in $C_{3}$ plants and $0-10\, ppm$ in $C_{4}$ plants. The reason for low compensation value for $C_{4}$ plants is the greater efficiency of $CO_{2}$-fixation through $PEP$-carboxylase. The optimum $CO_{2}$ concentration for $C_{4}$ plants is $360\, ppm$ and for $C_{3}$ plants, it is $450 \,ppm$.